The present invention relates to a projection type color image display device for combining an image on an image display device by a dichroic mirror, and projecting by a projection lens.
Previously, a projection type color image display device for obtaining a picture by projection lenses provided before cathode ray tubes (CRT) for displaying blue, green and red images has been proposed. On the other hand, a projection type color image display device comprising one projection lens has been also proposed. For example, a conventional projection type color image display device is shown in FIG. 4. In FIG. 4, the conventional projection type color image display device is composed of three liquid crystal panels with transmission type and one projection lens. The light emitted from a light source 101 is reflected by a reflector 102, and is separated into blue light, green light and red light respectively, by a first light-separating dichroic mirror 103, a second light-separating dichroic mirror 104, and a first reflecting mirror 105.
The separated lights are provided to a first liquid crystal panel with transmission type 106a, a second light crystal panel with transmission type 106b, and a third liquid crystal panel with transmission type 106c. In each liquid crystal panel, each color-light is modulated into light having an image. The color-lights modulated by the liquid crystal panels are combined by a first color-light-combining dichroic mirror 107, a second color-light-combining dichroic mirror 108, and a second reflecting mirror 109. The first color-light-combining dichroic mirror 107 and second color-light-combining dichroic mirror 108 are plates having parallel planes.
The combined color-light is magnified and projected by a projection lens 110, and a color picture is displayed on a screen 120.
In such a device, however, as the picture elements of the liquid crystal panel are advanced in definition, the picture quality deteriorates significantly. This is caused by the astigmatism occurring in the image of the color-light transmitted through the first color-light-combining dichroic mirror 107 and the image of the color-light passing through the second color-light-combining dichroic mirror 108, thereby extremely impairing the picture quality.
Since the first color-light-combining dichroic mirror 107 and second color-light-combining dichroic mirror 108 are disposed obliquely to the optical axis linking the liquid crystal panel and projection lens, such astigmatism occurs. FIG. 5 is a magnified view of essential parts of FIG. 4, and explains the onset mechanism of astigmatism. In FIG. 5, supposing the thickness of the first color-light-combining dichroic mirror 107 to be T, the refractive index of the substrate of the first color-light-combining dichroic mirror 107 to be N, the incident angle of the light transmitted through the first color-light-combining dichroic mirror 107 into the first color-light-combining dichroic mirror 107 to be X.sub.1, and the angle of approach in the first color-light-combining dichroic mirror 107 to be X.sub.2, the astigmatism Y is expressed in formula (1). ##EQU1##
From formula (1), the smaller the thickness of the first color-light-combining dichroic mirror 107, the smaller the value of astigmatism, but the thickness of the first color-light-combining dichroic mirror 107 cannot be reduced due to the following reasons.
If the first color-light-combining dichroic mirror 107 is thin, the shape may be deformed due to external force when assembling the first-color-combining dichroic mirror 107 into a support mechanism component (not shown), and the stress at this time causes the shape to change gradually. The first color-light-combining dichroic mirror 107 is composed by applying a dichroic coat on the substrate, but if the thickness is small, the substrate may be deformed by the heat in the process of applying the dichroic coat.
The first color-light-combining dichroic mirror 107 has the functions of transmitting the image-light from the first liquid crystal panel with transmission type 106a, and also reflecting the image-light from the second liquid crystal panel with transmission type 106b. Therefore, when reflecting the image-light from the second liquid crystal panel with transmission type 106b, in order that the reflected image may not be distorted, the surface of the first color-light-combining dichroic mirror 107 is demanded to have a flatness of high precision.
As for the second color-light-combining dichroic mirror 108, too, the thickness cannot be reduced due to the same reasons as in the first color-light-combining dichroic mirror 107.
As explained herein, it is impossible to reduce the thickness of the color-light-combining dichroic mirror, and the thickness of the color-light-combining dichroic mirror is large , thereby resulting in large astigmatism.
Due to the astigmatism, a deviation occurs between the focusing position of the projected image in the longitudinal direction and the focusing position in the lateral direction. Therefore, the longitudinal line and the lateral line cannot be focused at the same time. As a result, a clear image cannot be obtained, and only a blurry image is produced.
In FIG. 4, the image modulated only by the third liquid crystal panel with transmission type 106c, out of the first liquid crystal panel with transmission type 106a, second liquid crystal panel with transmission type 106b, and third liquid crystal panel with transmission type 106c, is led into the screen through reflection by the second reflecting mirror 109 and second color-light-combining dichroic mirror 108 only, so that astigmatism does not occur.
However, the image modulated by the second liquid crystal panel with transmission type 106b is transmitted through the second color-light-combining dichroic mirror 108, resulting in astigmatism. The image modulated by the first liquid crystal panel with transmission type 106a is transmitted through the first color-light-combining dichroic mirror 107 and second color-light-combining dichroic mirror 108, resulting in astigmatism. Therefore, when combining the images by projection of these images and adjusting the convergence, the multiplying factor in the longitudinal direction is different from the multiplying factor in the lateral direction owing to the presence of astigmatism. Consequently, the aspect ratio of the projected image varies depending on the value of astigmatism. As a result, it is difficult to match mutually the images projected by the three modulating means, that is, the first liquid crystal panel with transmission type 106a, second liquid crystal panel with transmission type 106b, and third liquid crystal panel with transmission type 106c, and a color image having color blurring at end portions is obtained, and a clear image cannot be reproduced.
In this way, when the light passes through the parallel plane plates disposed obliquely on the optical path, the optical axis on the optical path is included, and the focusing position on the first plane along the incident light and transmitted light includes its optical axis, and hence a position deviation to the focusing position on the second plane orthogonal to the first plane occurs, so that astigmatism occurs.
To solve such problems of color blurring and clouding in the projected image caused by astigmatism, a projection type image display device using color combining dichroic mirrors of a wedge shape is proposed. Generally, however, the dichroic mirrors for color-light-combination must have a high surface precision and precise spectral characteristic. Therefore, machining the color-light-combining dichroic mirrors into a wedge shape has a low yield of processing and has a very high manufacturing cost because the shape is complicated.
Besides, the dichroic mirrors for color-light-combination are composed by applying a dichroic coat on the substrate, and the substrate is exposed to high temperature when applying the dichroic coat on the substrate, which requires the use of substrates that can withstand high temperature, such as glass and ceramics. Using such glass or ceramics, machining of dichroic mirrors into a wedge shape requires a further higher cost.
On the other hand, to decrease astigmatism, it is proposed to place a parallel plane plate of glass for correcting astigmatism at a position of distortion to the color-light-combining dichroic mirror, and at a position having the same light incident angle as the color-light-combining dichroic mirror. In such a constitution, however, since the astigmatism correcting glass must be placed at a position of distortion to the color-light-combining dichroic mirror, it requires a wide space. Hence, the size of the assembled projection type image display device is large depending on the angle of distortion. For the material, glass is needed, and the glass processing cost is high.